Halfway to Newcastle to interview Eva Ibbotson, my mobile rings with a cranky message. "I do hope you're finding yourself some lunch, because I have absolutely nothing here to give you."

Inside her airy house, Ibbotson is sitting ramrod straight on a wing-backed chair, a papier-mache brick on the mantelpiece behind her labelled Writer's Block. "Well, you made it," she says. "I have nothing for you. I don't eat, I'm old. I'm tired and ill, this dreadful lupus, not the sort of thing you'll understand at all."

"Oh, I'm beat too," I say, falling into the chair opposite, as she eyes me. "I've just started chemotherapy for breast cancer. I can't even look at food today."

"Aha," she says, with a wicked grin, and, all trace of irritation gone now, her expression reads, "Trumped."

The bookshops are crammed with children's titles, but where is the quality in all that quantity? Amid the flurries of new titles, what do you choose? This summer, the problem is solved. Ibbotson has written a new novel, The Star of Kazan.

After the rollicking witch tales that made her name, Dial-A-Ghost, Which Witch, The Secret of Platform 13 (Harry Potter magic pre-Rowling), her writing took a new turn when her husband died six years ago. Her book Journey to the River Sea came from this recent period in her life and is a classic about identity, a theme in her work. The Star of Kazan returns to the Vienna of her early childhood. Ibbotson, as loving as she is sharp, lives now surrounded by children and grandchildren but her childhood was a recurring search for mothering.

The past few years have been hard. Just before her husband died, she contracted lupus, a rheumatic illness where the immune system turns on itself. "It makes you exhausted," she says, "so you're writing through a fog. It can be nasty, fatal, but mine is just a grinding-down version."

Then she cackles: "Well, considering I'm soon 80, it's a bit silly to expect to get by with nothing."

I tell her how I first came across The Star of Kazan - in a coffee shop in May, going through books that publishers send for this column, when our local independent bookseller came in and said: "Never mind all that, the best new book coming is The Star of Kazan."

"Oh, really?" Ibbotson looks up, bright-eyed. "Uh, well, that's nice. I did think, why am I suddenly writing about old Vienna when people want me to write about witches? But when my husband died and I got lupus, I thought, well, I will go on making jokes, but it would be nice to have a rest . . ."

In The Star of Kazan, a baby, Annika, is found by two women, a cook and housemaid for Viennese professors. They bring her up.

Ibbotson's parents were Jewish - "but we were the kind of Jews Hitler invented, the kind that had never been inside a synagogue" - who left Vienna when Eva was eight, in 1933, taking her first to Edinburgh, then London. Her parents had been separated since she was two, but becoming refugees together was a last - unsuccesful - stab at reuniting. "My mother was very Communist and theatre-oriented, and my father went the other direction into science. I was the only child, and it was a very early marriage that just didn't take. It was a time when politics were so important," she adds.

Her mother went to Paris, while she stayed in Edinburgh with her father - a scientist who pioneered artificial insemination - and a governess. Does she remember what her eight-year-old self felt? "I think I expected things to be chaotic. I was always wanting my mother to come, waiting and rushing out into the street, looking at people with hats like hers. Thirties hat, velour, very sort of clochey.

"She was wonderful when she came. But I think it was just too soon for her to be a mother. I don't think mother hood was very big in the 30s, with very committed left-wing people."

She loved English and learned fast. "I didn't speak German because my father shacked up with various English ladies - he was very successful with them; upper-class English ladies didn't expect to be talked to after you slept with them, so he had a string of very glamorous ladies - and my mother was with a Russian."

Her own marriage was also a release for her - from her education, honours physiology at Cambridge, which she was reading to become like her father. "Marriage and babies were the only way I could stop being a physiologist! Awful isn't it, all this women's lib which I supported, but honestly, the unutterable relief of finding a man willing to take me on and support me and let me have kids. I know it doesn't do to say so, but God, it was wonderful. And it stayed being wonderful for 49 years, and then the bugger died." Grief crosses her face like a shroud. Her other love has been writing, at first for magazines such as The Lady. She had always devoured Georgette Heyer alongside War and Peace, and decided to write high-quality romances. She made money swiftly, but her adult fiction is less successful here than it is on the continent.

But she is more than glad that the children's fiction has taken off. "You read what you've written," she says, "and you realise that something is still there. Because, you know, you see yourself tottering around, dropping china, having to go to bed at eight, but somehow something of your self remains, and you have written it."

Now 85, The Ogre of Oglefort's author tells Michelle Pauli about her late start as a children's author, why she always wants things to go well for her characters, and the number of aunts required to tell a story